Optical inspection method and optical inspection apparatus

ABSTRACT

In the conventional contaminant particle/defect inspection method, if the illuminance of the illumination beam is held at not more than a predetermined upper limit value not to give thermal damage to the sample, the detection sensitivity and the inspection speed being in the tradeoff relation with each other, it is very difficult to improve one of the detection sensitivity and the inspection speed without sacrificing the other or improve both at the same time. The invention provides an improved optical inspection method and an improved optical inspection apparatus, in which a pulse laser is used as a light source, and a laser beam flux is split into a plurality of laser beam fluxes which are given different time delay to form a plurality of illumination spots. The scattered light signal from each illumination spot is isolated and detected by using a light emission start timing signal for each illumination spot.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.12/819,528, filed on Jun. 21, 2010, now U.S. Pat. No. 7,990,530, whichis a Continuation of U.S. patent application Ser. No. 11/819,712, filedon Jun. 28, 2007, now U.S. Pat. No. 7,755,751, claiming priority ofJapanese Patent Application No. 2006-180632, filed on Jun. 30, 2006, theentire contents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to an optical inspection method and an opticalinspection apparatus for detecting a contaminant particle or a defect ofa sample to be inspected, such as a thin film substrate, a semiconductorsubstrate or a photomask by radiating light thereon, or in particular toan optical inspection method and an optical inspection apparatus havinga sensitivity or a throughput improved as compared with the conventionalmethod and apparatus.

In the production line of a semiconductor substrate or a thin filmsubstrate, the inspection is conducted for contaminant particlesattached on the surface of the semiconductor substrate or the thin filmsubstrate to monitor the dust generation in the manufacturing equipment.In the semiconductor substrate before forming a circuit pattern, forexample, the detection of fine contaminant particles or micro defects onthe order of not more than several tens of nm on the surface thereof isrequired. A conventional technique for detecting fine defects on thesurface of a sample such as a semiconductor substrate is described in,for example, U.S. Pat. No. 5,798,829, in which a focused laser lightflux is fixedly radiated on the surface of the semiconductor substrate(the illuminated area formed on the semiconductor substrate surface bythe laser light flux radiated is called an illumination spot), and thescattered light generated from a contaminant particle, if any, attachedon the semiconductor substrate is detected while rotating andtranslating the semiconductor substrate so that the whole surface of thesemiconductor substrate is inspected for a contaminant particle ordefect. For detecting the scattered light, an ellipsoidal mirror isused. The detection position on the semiconductor substrate is defined aprimary focus position of the ellipse and the light-receiving surface ofa photodetector is arranged at a secondary focus position. In this way,the scattered light generated from a contaminant particle is focused ata wide solid angle to detect even a fine contaminant particle. In thisconventional technique, only one laser light flux for illuminating thesemiconductor substrate corresponds to one incident angle, and only oneillumination spot is formed on the semiconductor substrate surface bythe particular laser light flux.

Another conventional technique is described in, for example,JP-A-2001-255278, in which a condenser lens and a photodetector arearranged at combined positions of a plurality of elevation angles andazimuthal angles with respect to the surface of the semiconductorsubstrate, and the scattered light focused by each condenser lens isdetected by the photodetector, so that a fine contaminant particle canbe detected in an advantageous direction conforming with thethree-dimensional radiation distribution characteristic of the scatteredlight from the particular fine contaminant particle. In this prior art,although two laser light fluxes for illuminating the semiconductorsubstrate exist for oblique and normal illumination, only one laserlight flux corresponds to one incident angle and also only oneillumination spot is formed on the semiconductor substrate surface bythe particular laser light flux.

With the semiconductor substrate (semiconductor wafer), the thin filmsubstrate and the photomask, the size of the contaminant particle ordefect requiring detection is sharply reduced with the increase inpackage density. In the case where the particle size of the contaminantparticle is so small as to follow the Rayleigh scattering, the scattersignal amount S obtained by the photodetector detecting the scatteredlight from the contaminant particle to be detected on a flat, smoothsample surface is generally proportional to the value of the right sideof the following equation:S∝(illuminance of illumination beam)×(size of contaminant particle tothe power of 6)×(illumination wavelength to the power of −4)×(collectionefficiency of scattered light detection optics)×(duration of scatteredlight)×(quantum efficiency of photodetector)×(gain of photodetector)where the noise level N for detection is generally substantiallyproportional to the value of the right side of the following equation:Square root of N∝(illuminance of illumination beam)×(area ofillumination spot)×(scattering efficiency of sample surface)

The following factors, therefore, have so far been well known to improvethe detection sensitivity of contaminant particles or defects:

(1) The illuminance of the illumination beam in the illumination spot isincreased to increase the strength of the scattered light.

(2) The wavelength of the illumination beam is shortened to increase thestrength of the scattered light.

(3) The numerical aperture of the focusing optics is increased toincrease the efficiency of focusing the scattered light.

(4) The performance of the photodetector such as quantum efficiency andS/N is improved.

(5) The background scattering is reduced by reducing the area of theillumination spot.

(6) The primary scanning rate of the sample stage is reduced to lengthenthe time for the contaminant particle or defect to pass through theillumination spot.

(7) The diameter of the illumination spot along the primary scanningdirection is increased to lengthen the time for a contaminant particleor defect to pass through the illumination spot.

Under the circumstances, however, it is not easy to improve thesensitivity even if these measures are taken, for the reasons describedbelow.

(1) An increased illuminance of the illumination beam causes the surfaceof the sample to absorb the energy of the illumination beam andincreases the surface temperature of the sample thereby increasing therisk of thermal damage to the sample.

(2) The wavelength of the available light source having an outputsuitable for detecting a contaminant particle or defect is limited andcannot be shortened to less than a certain limit.

(3) The collection efficiency of the scattered light emitted fromcontaminant particles or defects fails to reach more than 100%. Thefigure is generally about 50% in the prior art, and this value cannot bedoubled in the future.

(4) In the prior art, the quantum efficiency and S/N of thephotomultiplier tube used as a photodetector suitable for detecting weakscattered light have almost reached a theoretical limit and a furtherimprovement thereof is not expected.

(5) The area of the illumination spot can be effectively reduced at thesacrifice of a lengthened time required to inspect the whole surface ofthe sample to be inspected.

(6) A lower primary scanning rate, like in (5), leads to thedisadvantage of a longer time required to inspect the whole surface ofthe sample.

(7) The mere increase in the diameter of the illumination spot along theprimary scanning direction is not effective as it is offset by theincreased background scattering due to the increased area of theillumination spot, while a decreased diameter of the illumination spotalong the direction orthogonal to the primary scanning direction toprevent the illumination spot area from increasing, on the other hand,like in (5), disadvantageously lengthens the time required forinspection of the whole surface of the sample.

SUMMARY OF THE INVENTION

In view of this situation, the object of this invention is to provide anoptical inspection method and an optical inspection apparatus in whichthe detection sensitivity of contaminant particles or defects isimproved without sacrificing the time required to inspect the wholesurface of the sample, the time required for inspection of the wholesurface of the sample is reduced without sacrificing the detectionsensitivity of contaminant particles or defects, and both the detectionsensitivity of contaminant particles or defects and the time requiredfor inspection of the whole surface of the sample are improved. Thisinvention is intended to provide a technique for achieving this object.

In order to achieve the object described above, according to thisinvention, there is provided an optical inspection apparatus comprisinga means for forming not one but a plurality of illumination spots byradiating the illumination beam on the surface of the sample in order togenerate and detect the scattered light from a contaminant particle or adefect on the sample surface and detecting by isolating the scatteredlight signal generated from the plurality of the illumination spots foreach scattered light generated from each illumination spot. Morespecifically, there is provided an optical inspection apparatuscomprising a sample stage for moving a sample in accordance with apredetermined pattern, an illumination means for radiating the lightfrom a light source on the surface of the sample, and a light detectionmeans for detecting the light generated by the radiation of theillumination beam on the sample, wherein the illumination means includesa means for splitting a single light flux generated from the lightsource into a plurality of light fluxes and forming a plurality ofillumination spots in such a manner as to superpose at least a part ofthe loci plotted by the illumination spots generated in predeterminedspaced relation with each other on the sample by the illumination meansradiating the plurality of split light fluxes during the movement of thesample on the sample stage, and an optical signal isolation/detectionmeans for detecting by isolating the signal detected at different timepoints by the light detection means from the light generated at the sameposition on the sample from the plurality of the illumination spots.There is also provided an optical inspection method implemented by theaforementioned optical inspection apparatus. The light source may beeither a CW light source or a pulse light source for radiating the lightintermittently. To facilitate the timing adjustment for synthesizing thedetected light, however, the pulse light source is more preferable. Thisis by reason of the fact that the optical signal generated with thepassage of the same position of the sample through the plurality ofillumination spots is obtained from the photodetector, and the lightemission timing of the pulse light emission can be used as a timingsignal to synthesize the information for the same position. In the caseof the CW light source, the timing signal for synthesis can becalculated based on the speed of the sample stage. In this method, thesame point on the sample is inspected twice, and therefore, theinspection sensitivity can be improved. Also, the areas irradiated withthe plurality of illumination spots on the sample are arranged not to besuperposed one on another with the movement of the sample stage therebyto improve the throughput. Also in this case, the light emission timingof the pulse laser can be used to separate the optical signalinformation from the plurality of illumination spots. The layoutpatterns of the plurality of illumination spots can be large in numberas described in the embodiment later, and each layout pattern canproduce a unique effect. The photodetector is preferably aphotomultiplier tube having a high response speed. Depending on themoving speed of the sample stage, however, an ordinary optical elementsuch as a CCD can be used.

According to this invention, the detection sensitivity of contaminantparticles or defects can be improved without sacrificing the timerequired for inspecting the whole surface of the sample, or the timerequired for inspecting the whole surface of the sample can be improvedwithout sacrificing the detection sensitivity of contaminant particlesor defects. Further, both the detection sensitivity of contaminantparticles and defects and the time required for inspecting the wholesurface of the sample can be improved at the same time.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C are diagrams for explaining the effect of producing aplurality of illumination spots and arranging a plurality ofphotodetectors.

FIG. 2 is a diagram for explaining the technique for improving thedetection sensitivity of contaminant particles or defects withoutsacrificing the time required for inspection of the whole surface of asample according to this invention.

FIG. 3 is a diagram showing the light emission timing of a plurality ofillumination spots.

FIG. 4 is a diagram showing the light emission timing of the scatteredlight from each illumination spot.

FIG. 5 shows a circuit for isolating the signal using a plurality ofgate circuits.

FIG. 6 is a diagram for explaining the gate signal generated from thelight emission start timing signal.

FIG. 7 shows a circuit for isolating the signal using a plurality ofintegrators.

FIG. 8 is a diagram for explaining another light emission timing signalgenerating means.

FIG. 9 is a diagram for explaining a first technique for improving thetime required for inspection of the whole surface of the sample withoutsacrificing the detection sensitivity of contaminant particles ordefects according to the invention.

FIGS. 10A, 10B are diagrams for explaining a method of arranging aplurality of illumination spots.

FIG. 11 is a diagram for explaining a second technique for improving thetime required for inspection of the whole surface of the sample withoutsacrificing the detection sensitivity of contaminant particles ordefects according to the invention.

FIG. 12 is a diagram for explaining a technique to improve both thedetection sensitivity of contaminant particles or defects and the timerequired for inspection of the whole surface of a sample at the sametime according to this invention.

FIG. 13 is a diagram showing a configuration of a contaminantparticle/defect inspection apparatus according to an embodiment of theinvention.

DESCRIPTION OF THE INVENTION

This invention is explained in detail below.

The configuration in which the scattered light generated from a singleillumination spot is detected by a single photodetector as in the priorart is shown in FIG. 1A. A sample is mounted on a sample stage. Thesample stage moves in a combination of a translation movement forprimary scanning and another translation movement in the directionorthogonal to the first translation for secondary scanning or acombination of the rotational movement for primary scanning and thetranslation movement for secondary scanning. A contaminant particle 1 onthe surface of the sample is moved along a locus 2 by the primaryscanning of the sample stage. The contaminant particle 1, upon passagethrough an illumination spot 3 on the locus 2, generates the scatteredlight by the radiated light. This scattered light is condensed by ascattered light condenser lens 5, and detected and converted into ascattered light signal by a photodetector 7. Next, the configuration inwhich two illumination spots 3, 4 are arranged in juxtaposition not tobe superposed one on the other along the direction of primary scanningof the sample stage and the scattered light generated from the twoillumination spots are detected by a single photodetector 7 is shown inFIG. 1B. In the configuration of FIG. 1B, the contaminant particle 1,after passing through a first illumination spot 3, passes through asecond illumination spot 4, and therefore, two scattered light signalsare obtained from the photodetector 7. In the configuration of FIG. 1B,the sum of these two scattered light signals is calculated as a totalscattered light signal. Further, the configuration in which twoillumination spots 3, 4 are juxtaposed in a manner not to be superposedone on the other along the direction of primary scanning of the samplestage and two photodetectors 7, 8 detect the scattered light generatedfrom the two different illumination spots, respectively, is shown inFIG. 1C. In the configuration of FIG. 1C, the contaminant particle 1passes, after passing through the first illumination spot 3, passesthrough the second illumination spot 4, and therefore, two scatteredlight signals from the first and second photodetectors 7, 8 can beobtained. In the configuration of FIG. 1C, the sum of these twoscattered light signals is calculated as a total scattered light signal.

In FIGS. 1A to 1C, the size and area of the illumination spots, theilluminance of the radiated light in the respective illumination spotsand the collection efficiency of the condenser lenses 5, 6 are assumedto be identical with each other. In the configuration of FIGS. 1A to 1C,S/N of the total scattered light signal detected by moving the samecontaminant particle 1 on the same sample stage at the primary scanningspeed is compared as a value relative to S/N of the scattered lightsignal obtained in the configuration of FIG. 1A as unity. In theconfiguration of FIG. 1B, the size of the total scattered light signalis twice that of FIG. 1A. Also, in FIG. 1B, the background scatteredlight is generated from two illumination spots equivalent to FIG. 1A,and detected by a single photodetector 7, resulting in the backgroundscattered light strength twice as large as in FIG. 1A. As describedearlier, the noise increases substantially in proportion to the squareroot of the background scattered light strength, and therefore, thenoise in FIG. 1B is about √{square root over ( )}2 times as large as inFIG. 1A for each scattered light signal. In the case where the noisecharacteristic has no temporal correlation, the addition of twomeasurements of the signal containing the noises leads to the averagevalue (expected value) of the noise contained in the measurement notdouble but √{square root over ( )}2 times due to the averaging effect.Therefore, the size of the noise contained in the result of adding twoscattered light signals is twice that of FIG. 1A. As a result, therelative value of S/N of FIG. 1B with S/N of FIG. 1A as unity is nothingbut unity equivalent to that of FIG. 1A due to the fact that the totalscattered light signal is twice as large and so is the noise. Next, inthe configuration of FIG. 1C, the size of the total scattered lightsignal is twice as large as the scattered light signal of FIG. 1A. Also,in FIG. 1C, the background scattered light is generated from twoillumination spots equivalent to FIG. 1A. Since each photodetector candetect only the background scattered light from one illumination spot,however, the background scattered light strength detected by eachphotodetector is equal to that of FIG. 1A. For the same reason asdescribed in FIG. 1B, in the case where the scattered signals from thetwo photodetectors 7, 8 are added, the size of the noise contained inthe sum is √{square root over ( )}2 times as large as in FIG. 1A. As aresult, the relative value of S/N of FIG. 1C with S/N of FIG. 1A asunity is √{square root over ( )}2, both the total scattered light signaland the noise being twice as large. As compared with FIG. 1A, therefore,S/N is improved √{square root over ( )}2 times. It is easily understoodthat S/N can be improved √{square root over ( )}N times in the casewhere the configuration shown in FIG. 1C is assumed to involve not 2 butN illumination spots and N photodetectors as a general case.

In the configuration of FIGS. 1A to 1C, the inspection speed is equal,and therefore, it is understood from the foregoing description that thedetection sensitivity of the contaminant particle or defect can beimproved without sacrificing the time required for inspection of thewhole surface of the sample by the configuration in which a plurality ofillumination spots are juxtaposed not to be superposed one on anotheralong the direction of primary scanning of the sample stage and thescattered light signal from each of the plurality of the illuminationspots is isolated and detected. Also, it has been found that thisconfiguration can be realized by detecting the scattered light with thephotodetectors corresponding to the plurality of illumination spots,respectively.

Nevertheless, it is not easy to detect the scattered light by individualphoto detectors corresponding to the plurality of the illuminationspots, respectively, by reason of the facts that:

(1) In order to make sure that only the scattered light from acorresponding illumination spot enters each photodetector and noscattered light from the other illumination spots enters, ahigh-performance isolation imaging optics is required capable of imagingthe images of the respective illumination spots at sufficient distancefrom each other not to be superposed one on another.(2) Especially in the case where it is desired to detect the scatteredlight at a low elevation angle, the isolation imaging optics constitutesan oblique observation optics at low elevation angle, and therefore, theplurality of the illumination spots are distributed at spatial intervalssubstantially along the direction of focal depth of the optics, therebymaking it difficult to realize the optics capable of imaging theillumination spots sufficiently isolated from each other.(3) The provision of a plurality of combinations of the isolationimaging optic described in (1) and the photodetector including aphotomultiplier tube high in detection sensitivity corresponding to aplurality of illumination spots requires a large mounting volume.Especially, in the case where the scattered light is detected at eachposition of a plurality of elevation angles and a plurality of azimuthalangles combined, a plurality of combinations of the isolation imagingoptics described in (1) and the photomultiplier tube high in detectionsensitivity corresponding to the plurality of the illumination spots arerequired at respective positions of scattered light detection, resultingin a very large mounting volume. As a result, the parts, interferingwith each other spatially, may be impossible to arrange. A solution maybe to use small photodetectors or photodetectors each having a pluralityof pixels. Regrettably, however, this type of photodetector having asufficiently high detection sensitivity as compared with thephotomultiplier tube is not available and impossible to employ.

In order to obviate the aforementioned disadvantage and provide aneasy-to-realize method of improving the detection sensitivity, thisinvention provides a technique for isolating and detecting the scatteredlight from a plurality of illumination spots using a single but not aplurality of photodetectors.

This technique is explained in detail with reference to FIG. 3. Anillumination beam source 11 is constituted of a pulse laser forgenerating pulses by temporally repetitive pulse oscillation. The pulsesare generated at such time intervals that the light is emitted aplurality of times during the time when a contaminant particle 1 passesthrough an illumination spot by primary scanning. The light emitted fromthe light source 11 is split into two beams 21, 22 by a beam splitter12. The beams 21, 22 both finally enter a radiation lens 18 and formillumination spots 3, 4, respectively. Optical paths are formed in sucha manner that the beam 22 passes along a longer optical path than thebeam 21 in the meantime. In the case where the pulse oscillationinterval of the pulse laser source 11 is 10 ns, for example, the delaytime can be set to just one half of the time of the pulse oscillationinterval by setting the difference in optical path length to 1500 mm, asindicated by the following equation:Distance covered by light for 5 ns=(3×10¹¹)×(5×10⁻⁹) (mm)=1500 (mm)

The optical paths of the beams 21, 22 include beam splitters 14, 15 forretrieving a part of the beams 21, 22 as a light emission timing signalgenerating means to retrieve the light emission timing of each laserlight as a signal, and photodiodes 16, 17 for converting the time changesignal waveform of the partly retrieved light into an electrical signal.In order to correctly detect the light emission time difference betweenthe two illumination spots 3, 4, the two photodiodes 16, 17 are ofcourse required to be arranged equidistantly in the reverse way of theoptical paths of the beams 21, 22 from the illumination spots. The lightemission timing signal obtained from the photodiodes 16, 17 has thewaveform, for example, as shown in the two upper stages of FIG. 4.

Like in the configuration of FIG. 1B, the contaminant particle 1, afterpassing through the first illumination spot 3, passes through the secondillumination spot 4, so that the scattered light signal is obtainedtwice from the photodetector 7. The light source 11 is a pulse laser. Aslong as the time response speed of the photodetector 7 is sufficientlyhigh (this assumption is sufficiently realistic for the photomultipliertube), therefore, the temporal strength change of the scattered lightsignal generated while passing through each illumination spot assumesnot a continuous waveform but, as shown in FIG. 4, a discrete waveformcorresponding to the pulse oscillation of the light source 11. Asunderstood from FIG. 4, the illumination beam is radiated not strictlyat the same time on the first illumination spot 3 and the secondillumination spot 3, and therefore, the second scattered light from thefirst illumination spot 3 and the scattered light from the illuminationspot 4 are not generated at the same time point. According to thisinvention, the actual time point when the illumination beam is radiatedon each illumination spot can be accurately determined by the lightemission timing signal generating means described above. By isolatingthe output signal of the photodetector 7 using this light emissiontiming signal, therefore, the scattered light signals from a pluralityof illumination spots can be isolated and detected using a singlephotodetector. An example of the circuit configuration for thisoperation is shown in FIG. 5.

The output signal of the photodetector 7, after being amplified by apreamplifier 25, is distributed to two gate circuits 27. The on/offoperation of each gate circuit is controlled in accordance with thelight emission timing signals from the photodiodes 16, 17. Generally,however, as shown in the fifth stage of FIG. 6, the output signal of thephotodetector rises behind the light emission timing of the illuminationbeam and the frequency response speed is low as compared with the lightemission duration (for example, about 15 ps) of the pulse laser.Therefore, the pulse width of the output signal of the photodetector islonger than the light emission duration of the illumination beam. Inview of this, the light emission timing signals from the photodiodes 16,17 are given a predetermined delay time and a predetermined pulseduration by a waveform shaper 28. The output signal from the waveformshaper 28 assumes, for example, the waveforms shown in the third andfourth stages of FIG. 6. The gate circuits 27 are turned on/off by thegate signal from this waveform shaper 28, so that the isolated scatteredlight signals corresponding to each illumination spot shown in the sixthand seventh stages of FIG. 6 are generated from the output signal of thepreamplifier 25. Each scattered light signal thus isolated is thereafteramplified further by the amplifiers 26. It is thus understood that theuse of the circuit shown in FIG. 5 makes it possible to detect byisolating the scattered light signal corresponding to a plurality ofillumination spots, and each scattered light signal thus isolated is notaffected by the background light of the illumination spot other than thecorresponding one.

In the aforementioned case, the signal is isolated using the gatecircuits 27. A similar advantage is achieved, however, by aconfiguration shown in FIG. 7, in which integrators 29 are arranged inplace of the gate circuits 27, and the integrating operation of theintegrators 29 is controlled by the signal of the waveform shaper 28.

In a combined configuration of the optics shown in FIG. 2 and thecircuit shown in FIG. 5 or 7, assuming that sum of the two scatteredlight signals is calculated as the total scattered light signal, thesize of the total scattered light signal is twice as large as thescattered light signal for the configuration shown in FIG. 1A. Also,each scattered light signal isolated by the circuit of FIG. 5 or 7 isaffected by the background scattered light from only one illuminationspot, and therefore, the noise contained in each scattered light signalis equal to the case of the configuration of FIG. 1A, and the size ofthe noise contained in the sum of the two scattered light signal is√{square root over ( )}2 times as large as in FIG. 1A. As a result, inview of the fact that the total scattered light signal is twice and thenoise √{square root over ( )}2 times as large, the value of S/N of theconfiguration shown in FIGS. 3 and 5 relative to S/N of FIG. 1A as unityis improved √{square root over ( )}2 times as large. This is aseffective as when the two photodetectors are used as shown in FIG. 1C.Although the configuration of FIGS. 2, 5, 7 is explained with twoillumination spots, it will be easily understood that S/N can beimproved √{square root over ( )}N times in a generalized case where thenumber of illuminations is N instead of 2. In the aforementioned casewhere N is 2, the final result is determined as a simple sum. In thecase where N is larger than 2, on the other hand, the final scatteredlight signal may be obtained by more complicated statistical process.Also, in spite of the foregoing explanation that the light emitted whenthe contaminant particle 1 passes through the illumination spot isassumed to be the scattered light, this light may be generalized as thescattered/diffracted/reflected light. Further, instead of thecontaminant particle existing on the surface of a sample, the inventionis equally applicable to defects such as a scratch or a crystal defectother than the contaminant particle existing in the neighborhood of aswell as on the surface of the sample.

As described above, the optical inspection apparatus includes a pulselaser source for oscillating the temporally repetitive pulses, whereinone light flux emitted from the pulse laser source is split into aplurality of light fluxes, after which a different delay time is givento each light flux. These light fluxes are radiated at a plurality ofpositions substantially aligned at predetermined spatial intervals in amanner not to be superposed one on another substantially along the samedirection as primary scanning of the sample stage thereby to form aplurality of illumination spots on the surface of the sample. The photodetector to which the scattered/diffracted/reflected light generatedfrom the plurality of the illumination spots are adapted to entersubstantially at the same time detects the particularscattered/diffracted/reflected light. The output signal from thephotodetector is controlled based on the light emission start timingsignal from the light emission start timing signal generating meansindicating the light emission start timing of each of the plurality oflight fluxes given mutually different delay time. In this way, thescattered/diffracted/reflected light signal due to thescattered/diffracted/reflected light from each of the plurality ofillumination spots is isolated and detected. With this configuration,the detection S/N is improved for an improved detection sensitivity ofthe contaminant particle or defect without sacrificing the time requiredfor inspection of the whole surface of the sample.

Although the foregoing description of the prior art deals with atechnique for detecting the scattered/diffracted/reflected light from aplurality of directions with a plurality of elevation angles and aplurality of azimuthal angles combined, the technique according to theinvention described above is applicable also to the detection method foreach of a plurality of directions with a plurality of the elevationangles and a plurality of azimuthal angles combined. The direction inwhich the illumination beam is incident to the sample surface may ofcourse be either oblique or normal.

In the circuit shown in FIG. 5 or 7, the signal is isolated by use ofthe light emission timing signal from the light emission timing signalgenerating means shown in FIG. 2. In the optics shown in FIG. 2, thislight emission timing is optically monitored on two optical paths ofradiated beams. As shown in FIG. 8, however, a part of the lightspecularly reflected from the two illumination spots may alternativelybe optically monitored. Also, as described above, the signal of thephotodetector is lower in frequency response speed, and therefore, onlythe light emission start timing information of the aforementioned timingsignal is required, and the means used for the detection gate functionas described in JP-A-2000-338048 which faithfully grasps the emittedlight waveform of the actual pulse laser is not required. This lightemission timing signal, therefore, may be replaced by a light emissionstart timing signal capable of faithfully reproducing only theinformation on the light emission start timing of each light emissionpulse. Further, the use of the light emission start timing aloneeliminates the need of optically monitoring the actual light emission ofthe laser beam as in FIGS. 2 and 8, and the light emission start timingsignal may be generated based on the light emission sync signal outputfrom the pulse laser 11 or the light emission control signal applied tothe pulse laser 11 from an external source to control the pulse laser11. In such a case, the delay time of the second illumination spothaving the light emission timing behind the first illumination spot canbe determined in advance by calculating the difference between the twooptical path lengths.

Next, the technique according to the invention in which the timerequired for inspecting the whole surface of the sample is improvedwithout sacrificing the detection sensitivity of a contaminant particleor a defect is explained with reference to FIG. 9. The configuration ofFIG. 9 is different from that of FIG. 2 only in the spatial allocationof two illumination spots, and the operation, being identical, is notexplained in detail. In the configuration of FIG. 2, the twoillumination spots are substantially aligned substantially along thesame direction as the primary scanning of the sample stage atpredetermined spatial intervals with each other in a manner not to besuperposed one on the other. In FIG. 9, on the other hand, the twoillumination spots are substantially aligned closely in a manner not tobe superposed one on the other in substantially the same direction asthe secondary scanning of the sample stage. In this configuration, thescattered light signals corresponding to the illumination spots 3, 4 canbe isolated and detected from the output signal of the photodetector 7by exactly the same function as explained in the configuration of FIG.2. It is understood that each of these scattered light signals, in boththe scattered light strength and the noise level contained, is equal tothe configuration shown in FIG. 1A. As a result, with thisconfiguration, the width of the two illumination spots along thedirection of secondary scanning is twice as large as in FIG. 1B. With adetection sensitivity equal to FIG. 1A, therefore, the inspection speedcan be improved to twice as high. Although the configuration of FIG. 9is explained on the assumption of two illumination spots, it will beeasily understood that the inspection speed is improved to N times ashigh in the case where the number of illumination spots is not 2 but Nas a general case.

As described above, the optical inspection apparatus according to thisembodiment, having a pulse laser source for oscillating temporallyrepetitive pulses, is so configured that one light flux emitted from thepulse laser source is splitted into a plurality of light fluxes, afterwhich each light flux is given a different delay time, and a pluralityof illumination spots are formed by radiating the light at a pluralityof positions substantially aligned closely in a manner not to besuperposed one on another in substantially the same direction as thesecondary scanning of the sample stage. Then, a photo detector enteredsubstantially at the same time by the scattered/diffracted/reflectedlight generated from the plurality of the illumination spots detects thescattered/diffracted/reflected light, and the output signal of thephotodetector is controlled based on the light emission start timingsignal from the light emission start timing signal generating meansarranged inside or outside the illumination means for indicating thelight emission start timing of each of a plurality of light fluxes givendifferent delay time. In this way, the scattered/diffracted/reflectedlight signal due to the scattered/diffracted/reflected light from theplurality of the illumination spots are isolated and detected. Thus,without sacrificing the detection sensitivity, the time required forinspecting the whole surface of the sample can be improved.

Although the conventional technique described above represents a case inwhich the scattered/diffracted/reflected light is detected from aplurality of directions with a plurality of elevation angles and aplurality of azimuthal angles combined, the aforementioned techniqueaccording to the invention is applicable to a method of detection ineach of a plurality of directions with the plurality of elevation anglesand the plurality of azimuthal angles combined. The direction in whichthe illumination beam is incident to the sample surface may of course beeither oblique or normal.

In the case described above, a plurality of illumination spots aresubstantially aligned closely not in superposition with each other insubstantially the same direction as the secondary scanning of the samplestage. In the case where it is not desired to use the outer edge of eachillumination spot low in illuminance or to improve the detectionaccuracy by detecting each contaminant particle by two primary scans,then, as shown in FIG. 10A, a plurality of illumination spots aresuitably superposed one on another at a predetermined ratio. In thesuperposed area of the illumination spots, however, the energy of theillumination beams of the two illumination spots is received, andtherefore, temperature increases to such an extent that the sample maybe thermally damaged considerably. In view of this, the plurality ofillumination spots are arranged displaced at predetermined intervals inthe direction of primary scanning as shown in FIG. 10B. Thus, while thesuperposition of illumination spots is obviated, the effect of thermaldamage is avoided. At the same time, the loci plotted by theillumination spots by primary scanning are superposed one on another ata predetermined ratio, and therefore, the original object describedabove can be achieved. In other words, this arrangement is equivalent tothe arrangement of illumination spots substantially in alignment along apredetermined direction at an angle to the direction of primary scanningof the sample stage between the translation direction and the orthogonaldirection. The configuration in which a part of the plurality ofillumination spots shown in FIG. 9 is changed partially into thearrangement of illumination spots as shown in FIG. 10B is shown in FIG.11. The advantage obtained by this configuration other than thatobtained from the superposition of the illumination spots is identicalwith that shown in FIG. 9, and therefore not described again.

Further, the arrangement of illumination spots shown in FIG. 2 may becombined with that shown in FIG. 11. Specifically, assuming that M and Nare integers, a plurality of illumination spots in the number of M×N aredivided into N groups of M illumination spots, the M illumination spotsin each of the N groups are substantially aligned in substantially thesame direction as the primary scanning of the sample stage on the samplesurface. At the same time, the N groups each including M illuminationspots are substantially aligned in a predetermined direction at an angleto the primary scanning of the sample stage between the translationdirection and the orthogonal direction. An example of this configurationwith M=2 and N=2 is shown in FIG. 12. The beam splitter and thephotodiode making up the light emission start timing generating means,though not described, are arranged at positions equidistantly locatedalong the reverse optical path from each illumination spot in eachoptical path of the four radiation beams shown in FIG. 12.

As will be readily understood, this configuration can produce theadvantages of the configurations of both FIG. 2 and FIG. 11.Specifically, the detection sensitivity of the contaminant particle ordefect and the time required for inspecting the whole surface of thesample are improved at the same time.

Although FIG. 12 employs the method of arrangement in the configurationof FIG. 11 to arrange the illumination spots in the direction ofsecondary scanning, the illumination spots may alternatively be arrangedclosely in the direction of secondary scanning as shown in theconfiguration of FIG. 9.

A contaminant particle/defect inspection apparatus using the contaminantparticle/defect detection method according to an embodiment of thisinvention is shown in FIG. 13. A semiconductor wafer 100 making up asample to be inspected is attached by vacuum to a chuck 101, which inturn is mounted on a sample stage 102 including a rotation stage 103, atranslation stage 104 and a Z stage 105. An illumination/detectionoptics 110 arranged above the semiconductor wafer 100 is the opticalsystem shown in FIG. 11. Specifically, the light source 11 of theillumination beam is comprised of a pulse laser for oscillating pulsestemporally repetitively. The pulses are oscillated at such timeintervals that light is emitted a plurality of times within the timewhile the contaminant particle 1 passes through the illumination spot byprimary scanning. The light emitted from the light source 11 is splitinto two beams 21, 22 by a beam splitter 12. The beams 21, 22 both entera radiation lens 18 finally to form the illumination spots 3, 4,respectively. In the process, however, the optical paths are soconfigured that the delay time of the beam 22 behind the beam 21corresponds to just one half of the time of the pulse oscillationinterval. The optical paths of the beams 21, 22 each have arrangedthereon beam splitters 14, 15 for retrieving a part of the beams 21, 22as a light emission start timing generating means for retrieving thelight emission start timing of each laser beam as a signal andphotodiodes 16, 17 for converting the temporal change waveform of thelight partly retrieved by the beam splitters 14, 15 into an electricalsignal. In order to correctly detect the light emission time differencebetween the two illumination spots 3, 4, the two photodiodes 16, 17 arearranged equidistantly in reverse way of the optical path of the beams21, 22 from the illumination spot side. Each illumination beam isirradiated onto the surface of the semiconductor wafer 100 in obliqueincidence substantially with Brewster angle of crystal Si. Also, thecondenser lens 5 is configured to focus the scattered light at a lowelevation angle to capture the scattered light efficiently for finecontaminated particles subjected to Rayleigh scattering. In thisconfiguration, the contaminated particle 1, after passing through thefirst illumination spot 3, passes through the second illumination spot4, so that the scattered light signal is produced twice from thephotodetector 7. The output signal of the photodetector 7 is processedby a circuit configured as shown in FIG. 7. Specifically, the outputsignal of the photodetector 7, after being amplified by a preamplifier25, is distributed to two integrators 29. The integrating operation ofeach integrator is turned on/off by the integration control signalproduced by giving a predetermined delay time and a predetermined pulseduration to the light emission start timing signal from the photodiodes16, 17 through a waveform shaper 28, thereby isolating the scatteredlight signal corresponding to each illumination spot. Each scatteredlight signal thus isolated is further amplified by an amplifier 26. Byuse of the circuit of FIG. 7 in this way, the scattered light signalscorresponding to a plurality of illumination spots can be isolated anddetected from the output of a single photodetector 7. Each scatteredlight signal thus isolated is not affected by the background light ofthe illumination spots other than the corresponding ones. As a result,the contaminated particle/defect inspection apparatus according to anembodiment of the invention shown in FIG. 13 can improve the timerequired for inspection of the whole surface of the sample withoutsacrificing the detection sensitivity.

Instead of two illumination spots employed in the configuration of FIG.13, illumination spots in the number of N not less than 2 can be used,in which case the inspection speed is improved N times.

Although the embodiment of the invention described above has only onedirection in which the scattered light is detected, the scattered lightmay alternatively be detected from a plurality of directions byarranging the aforementioned detection system in each of a plurality ofdirections with a plurality of elevation angles and a plurality ofazimuthal angles combined. The illumination beam may of course beradiated on the sample surface either obliquely or normally.

Also, in spite of the method of arranging a plurality of illuminationspots shown in FIG. 11, the arrangement shown in any of FIGS. 2, 9, 12may of course alternatively be used.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An inspection apparatus comprising: an irradiation unit whichirradiates a substrate with pulse light; a switching unit which executesswitching as a function of emission of said pulse light; a detectionunit which detects light from said substrate as a function of saidswitching; and an emission-timing-signal generating unit which detectssaid pulse light, and generates an emission-timing-signal of said pulselight, wherein said switching unit is configured to use saidemission-timing-signal as input for said switching.
 2. The inspectionapparatus according to claim 1, further comprising: a wave generatingunit which generates a delay signal, wherein said switching unit isconfigured to use said delay signal as further input for said switching.3. The inspection apparatus according to claim 1, further comprising: awave generating unit which generates continuous signals, wherein saidswitching unit is configured to further use said continuous signals asfurther input for said switching.
 4. The inspection apparatus accordingto claim 1, wherein said switching unit includes at least one of a gatecircuit, and an integral circuit.
 5. The inspection apparatus accordingto claim 1, wherein said detection unit includes aphotoelectric-conversion device for detecting said light from saidsubstrate, wherein the inspection apparatus further comprises: a firstamplifier which amplifies a signal detected by saidphotoelectric-conversion device, wherein said switching unit switches asignal amplified by said first amplifier.
 6. The inspection apparatusaccording to claim 5, further comprising: a second amplifier whichamplifies a signal switched by said switching unit.
 7. The inspectionapparatus according to claim 1, wherein said irradiation unit isconfigured to form an illumination spot larger than a defect on saidsubstrate, and irradiate said substrate with said pulse light aplurality of times while said spot passes a defect on said substrate. 8.An inspection method comprising steps of: by way of an irradiation unit,irradiating a substrate with pulse light; by way of a switching unit,executing switching as a function of emission of said pulse light;detecting light from said substrate as a function of said switching;detecting said pulse light with an emission-timing-signal generatingunit and generating an emission-timing-signal of said pulse light; andinputting said emission-timing-signal into said switching unit duringexecution of the switching step.
 9. The method according to claim 8,further comprising: generating a delay signal by a wave generating unit;and inputting said delay signal into said switching unit duringexecution of said switching step.
 10. The method according to claim 8,further comprising: generating continuous signals by a wave generatingunit; and inputting said continuous signals into said switching unitduring execution of said switching step.
 11. The method according toclaim 8, wherein said switching unit includes at least one of a gatecircuit, and an integral circuit.
 12. The method according to claim 8,wherein the detecting step includes a detection unit including aphotoelectric-conversion device for detecting said light from saidsubstrate, wherein the method further comprises: by way of a firstamplifier, amplifying a signal detected by said photoelectric-conversiondevice; and by way of said switching unit, switching a signal amplifiedby said first amplifier.
 13. The method according to claim 12, furthercomprising: by way of a second amplifier, amplifying a signal switchedby said switching unit.
 14. The method according to claim 8, furthercomprising: by way of said irradiation unit, forming an illuminationspot larger than a defect on said substrate, and irradiating saidsubstrate with said pulse light a plurality of times while said spotpasses a defect on said substrate.
 15. The inspection apparatusaccording to claim 1, wherein said detection unit converts said lightinto an electrical signal, and outputs said electrical signal as afunction of said switching.
 16. The inspection apparatus according toclaim 1, further comprising: a dividing unit which divides said pulselight into a first divided pulse light and a second divided pulse light,wherein: said first divided pulse light proceeds to said substrate, andsaid emission-timing-signal generating unit detects said second dividedpulse light, and generates said emission-timing-signal.
 17. Theinspection apparatus according to claim 1, wherein saidemission-timing-signal generating unit detects reflection light causedby irradiating said substrate with said pulse light, and generates saidemission-timing-signal.
 18. The inspection apparatus according to claim1, wherein said emission-timing-signal generating unit is a diode. 19.The inspection apparatus according to claim 1, wherein: said irradiationunit forms a first irradiated area and a second irradiated area on saidsubstrate at different times; and said emission-timing-signal generatingunit detects a first light caused by forming said first irradiated area,and a second light caused by forming said second irradiated area. 20.The inspection apparatus according to claim 19, further comprising: astage which moves the substrate linearly, wherein said irradiation unitforms said first irradiated area and said second irradiated area along adirection that said stage moves the substrate linearly.
 21. Theinspection apparatus according to claim 1, wherein saidemission-timing-signal expresses start of said pulse light emission. 22.An inspection apparatus comprising: a supply unit which supplies anobject with pulse radiation; a radiation-timing-signal generating unitwhich detects said pulse radiation, and outputs aradiation-timing-signal regarding said pulse radiation; a switching unitwhich executes switching as a function of the supplied pulse radiation,said switching unit is configured to use said radiation-timing-signal asinput for said switching; and a detection unit which detects a reactioncaused by the supplied pulse radiation, and outputs a detected signal asa function of said switching.
 23. The inspection apparatus according toclaim 22, wherein: said pulse radiation is pulse light, and saidreaction is light from said object.
 24. The inspection apparatusaccording to claim 22, further comprising: a wave generating unit whichgenerates a second signal, wherein said switching unit is configured touse said second signal as further input for said switching.
 25. Theinspection apparatus according to claim 24, wherein said second signalis a delay signal.
 26. The inspection apparatus according to claim 24,wherein said second signal is a continuous signal.
 27. The inspectionapparatus according to claim 22, wherein said switching unit includes atleast one of a gate circuit, and an integral circuit.
 28. The inspectionapparatus according to claim 22, wherein: said detection unit includes aphotoelectric-conversion device for detecting said reaction, theinspection apparatus comprises a first amplifier which amplifies asignal detected by said photoelectric-conversion device, and saidswitching unit switches a signal amplified by said first amplifier. 29.The inspection apparatus according to claim 26, further comprising: asecond amplifier which amplifies a signal switched by said switchingunit.
 30. The inspection apparatus according to claim 22, wherein saidsupply unit is configured to: form a radiated area larger than a defecton said object, and supply said radiated area with said pulse radiationa plurality of times.
 31. The inspection apparatus according to claim22, wherein said detection unit converts said reaction into anelectrical signal, and outputs said electrical signal as a function ofsaid switching.
 32. The inspection apparatus according to claim 22,further comprising: a dividing unit which divides said pulse radiationinto first divided pulse radiation and second divided pulse radiation,wherein: first divided pulse radiation is directed to said object, andsaid radiation-timing-signal generating unit detects said second dividedpulse radiation, and generates said radiation-timing-signal.
 33. Theinspection apparatus according to claim 22, wherein saidradiation-timing-signal generating unit detects reflection light causedby supplying said object with said pulse radiation, and generates saidemission-timing-signal.
 34. The inspection apparatus according to claim22, wherein said radiation-timing-signal generating unit is a diode. 35.The inspection apparatus according to claim 22, wherein: said supplyunit forms a first radiated area and a second radiated area on saidobject at different times, and said radiation-timing-signal generatingunit detects a first reaction caused by forming said first radiatedarea, and a second reaction caused by forming said second radiated area.36. The inspection apparatus according to claim 35, further comprising:a stage which moves said object linearly, wherein said irradiation unitforms said first radiated area and said second radiated area along adirection that said stage moves said object linearly.
 37. The inspectionapparatus according to claim 22, wherein said radiation-timing-signalexpresses start of said pulse radiation generation.
 38. The methodaccording to claim 8, further comprising: converting, by way of adetection unit, said light into an electrical signal, and outputs saidelectrical signal as a function of said switching.
 39. The methodaccording to claim 8, further comprising: dividing said pulse light intoa first divided pulse light and a second divided pulse light, wherein:said first divided pulse light is directed to said substrate, and saidemission-timing-signal generating unit detects said second divided pulselight, and generates said emission-timing-signal.
 40. The methodaccording to claim 8, wherein said emission-timing-signal generatingunit detects reflection light caused by irradiating said substrate withsaid pulse light, and generates said emission-timing-signal.
 41. Themethod according to claim 8, wherein said emission-timing-signalgenerating unit is a diode.
 42. The method according to claim 8, furthercomprising: by way of the irradiation unit, forming a first irradiatedarea and a second irradiated area on said substrate at different times;and by way of the emission-timing-signal generating unit, detecting afirst light caused by forming said first irradiated area, and a secondlight caused by forming said second irradiated area.
 43. The methodaccording to claim 42, further comprising: moving the substrate linearlyby way of a stage, wherein said irradiation unit forms said firstirradiated area and said second irradiated area along a direction thatsaid stage moves the substrate linearly.
 44. The method according toclaim 8, wherein said emission-timing-signal initiates start of saidpulse light emission.
 45. An inspection method comprising steps of:supplying, by way of a supply unit, an object with pulse radiation;detecting said pulse radiation, by way of a radiation-timing-signalgenerating unit, and outputting a radiation-timing-signal regarding saidpulse radiation; executing switching, by way of a switching unit, as afunction of the supplied pulse radiation, said switching unit isconfigured to use said radiation-timing-signal as input for saidswitching; and detecting, by way of a detection unit, a reaction causedby the supplied pulse radiation, and outputting a detected signal as afunction of said switching.
 46. The method according to claim 45,wherein: said pulse radiation is pulse light, and said reaction is lightfrom said object.
 47. The method according to claim 45, furthercomprising: a wave generating unit which generates a second signal,wherein said switching unit is configured to use said second signal asfurther input for said switching.
 48. The method according to claim 47,wherein said second signal is a delay signal.
 49. The method accordingto claim 47, wherein said second signal is a continuous signal.
 50. Themethod according to claim 45, wherein said switching unit includes atleast one of a gate circuit, and an integral circuit.
 51. The methodaccording to claim 45, further comprising: detecting said reaction witha photoelectric-conversion device of the detecting unit; amplifying,with a first amplifier, a signal detected by saidphotoelectric-conversion device; and switching a signal amplified bysaid first amplifier by a switching unit.
 52. The method according toclaim 49, further comprising: amplifying a signal switched by saidswitching unit by a second amplifier.
 53. The method according to claim45, further comprising: forming a radiated area larger than a defect onsaid object by said supply unit; and supplying said radiated area withsaid pulse radiation a plurality of times.
 54. The method according toclaim 45, further comprising: converting said reaction into anelectrical signal by said detection unit; and outputting said electricalsignal as a function of said switching.
 55. The method according toclaim 45, further comprising: dividing, by way of a dividing unit, saidpulse radiation into first divided pulse radiation and second dividedpulse radiation, wherein: first divided pulse radiation is directed tosaid object, and said radiation-timing-signal generating unit detectssaid second divided pulse radiation, and generates saidradiation-timing-signal.
 56. The method according to claim 45, furthercomprising: detecting mirror reflection light caused by supplying saidobject with said pulse radiation by said radiation-timing-signalgenerating unit; and generating said emission-timing-signal.
 57. Themethod according to claim 45, wherein said radiation-timing-signalgenerating unit is a diode.
 58. The method according to claim 45,further comprising: forming, by said supply unit, a first radiated areaand a second radiated area on said object at different times; anddetecting, by said radiation-timing-signal generating unit, a firstreaction caused by forming said first radiated area, and a secondreaction caused by forming said second radiated area.
 59. The methodaccording to claim 45, further comprising; moving the object linearly bya stage; and forming, by said irradiation unit, said first radiated areaand said second radiated area along a direction that said stage movessaid object linearly.
 60. The method according to claim 45, wherein saidradiation-timing-signal initiates start of said pulse radiationgeneration.